The further reduction of transmission loss has been called for with respect to optical fibers used as an optical transmission line for long-haul communication. The known factors for attenuation of an optical fiber are absorption loss, scattering loss, loss due to structural disorder, and external loss. Particularly in the communication wavelength band (around 1.3 μm and 1.5 μm), the Rayleigh scattering loss is a main factor of attenuation. In an optical fiber made of silica glass, in order to achieve propagation due to total reflection of light, refractive indexes for a core and a cladding are appropriately designed, and the control of such refractive index is accomplished by adding dopants, such as germanium (Ge), chlorine (Cl) and fluorine (F).
JP2002-082250A discloses a low-nonlinearity single-mode optical fiber which includes a first center core layer, a second center core layer formed around the perimeter thereof, and a cladding layer formed around the perimeter of the second center core layer. This optical fiber has relationship of n1>n2, n2≤n0, and n1>n0, wherein the average refractive index of the above-mentioned layer is n1, n2, and n0, respectively, and the desired relative refractive index differences are acquired by doping Ge, etc. From a viewpoint of reliability for a long period of time, this optical fiber has improved characteristics: the generation of glass defect is controlled; no attenuation increase due to combination of glass defect and hydrogen is caused; and the durability under hydrogen is improved.
JP2002-148466A discloses an optical fiber in which a cladding region is formed around the perimeter of a core region. In this optical fiber, the ratio of the average viscosity of the core region to the average viscosity of the whole region consisting of the core region and the cladding region is 2.5 or less, and the Rayleigh scattering loss coefficient is 95% or less of a predetermined standard value A0, where A0 (dB/(km·μm4))=0.85+0.29×[Ge], and [Ge] is the amount of germanium added to the core region, as expressed by % in terms of the relative refractive-index difference with respect to pure SiO2.
WO2009/107260 discloses an optical fiber in which an outer core layer is formed around the perimeter of the center core region and a cladding layer is formed around the perimeter of the outer core layer. In this optical fiber, Ge is added to the center core region, fluorine is added to the outer core layer, the cladding layer being made of pure silica glass, and Δ1 is in the range of 0.30% to 0.35% while Δ2 is in the range of −0.1% to −0.04%, whereas |Δ1|/|Δ2| is in the range of 2.5 to 7.5, wherein Δ1 represents the relative refractive index difference of the center core region to the cladding layer and Δ2 represents the relative refractive index difference of the outer core layer to the cladding layer. This optical fiber is such that the bending loss, which is an external loss, is improved.